One hallmark of heart failure (HF) in a subject is congestion, more commonly referred to as fluid retention. Reasons for fluid retention are complex and multifactorial. Fluid retention or accumulation in the lungs or other respiratory organs (e.g., the trachea, the bronchi, etc.) is referred to as Pulmonary Edema (PE). In certain examples, fluid can accumulate in extracellular spaces, such as the spaces between lung tissue cells, commonly referred to as interstitial pulmonary edema. If left untreated, fluid can move from the extracellular space into the lung air-sacs, commonly referred to as alveolar pulmonary edema. One of the most common systems of heart failure is shortness of breath resulting from pulmonary congestion.
Although subjects with chronic heart failure can have varying levels of pulmonary congestion, they can be clinically stable most of the time without debilitating symptoms such as shortness of breath. However, fundamental changes in a disease state or other factors, such as non-compliance with diet, medication, etc., can worsen the heart failure condition, resulting in increased pulmonary congestion or edema. Although advanced pulmonary edema can be a serious or potentially fatal medical condition, it can be averted by treatment if detected early. Accordingly, various chronic ambulatory sensors and associated methods have been developed to detect the early onset of pulmonary and thoracic congestion that including cardiopulmonary pressure sensors or thoracic or lung fluid sensors. In certain examples, chronic ambulatory sensors can be used independently or in combination to detect the early onset of pulmonary congestion.